In order to obtain increased two-phase flow heat transfer performance, heat transfer tubes have been provided with surface enhancements on their inner surfaces. The higher heat transfer performance of an internally enhanced tube as compared to a smooth tube can be utilized to reduce the size of heat exchangers which, in turn, provides the advantages of increased energy efficiency, reduced noise levels, and cost reductions in air conditioning and refrigeration equipment.
An early form of internally enhanced tube was the helical type which can be characterized as numerous continuous fins extending spirally along the tube axis. An example of such a helical tube is disclosed in U.S. Pat. No. 4,658,892. The fins typically are formed by an extrusion process and are substantially trapezoidal in cross-sectional shape with the larger end at the junction of the fin and the tube wall. This tube improves refrigerant evaporation heat transfer up to about two times that of the performance of a corresponding smooth tube. It also has one and one-half to two times the performance of the smooth tube in condensation. On the other hand, refrigerant flow pressure drop, which is not desired, is increased only about 30% to 50% in both evaporation and condensation.
Thereafter an axial internally enhanced tube was developed, which is a variation of the helical internally enhanced tube, with the helical angle of the fins being 0 degrees. This tube typically has more fins than the helical type and has more surface area. The axial internally enhanced tube has two-phase flow heat transfer performance similar to that of the helical tube in most practical flow rates, but provides significantly lower refrigerant pressure drop.
Crosshatch internally enhanced tubing is available currently in the air conditioning and refrigeration industry. It employs the axial or helical tube as its first enhancement and a cross notch of the continuous fins as the second enhancement to provide a relatively more complicated surface structure. Instead of continuous fins, like those in the helical and axial internally enhanced tubes, small segments of fins are provided on the tube inner surface. The crosshatch internally enhanced tube significantly increases condensation performance, i.e. about 35 percent, while providing a similar evaporation performance compared with the helical tube. The pressure drop of the crosshatch tube is slightly higher than that of the helical tube and significantly higher than that of the axial tube. Examples of crosshatch internally enhanced tubing are shown and described in U.S. Pat. Nos. 5,332,034 and 5,458,191.